It always depends on the wrestler. However it tends to be somewhere in the early to mid thirties. Go earlier and they have the physical capability, but mentally they're on the green side. Go later and they've got a top notch veteran's mind, but their physical capabilities are more limited.
It all depends on how old they are when they start. I'd say around 35 because they're old enough to have experience, but young enough to usually not have the health issues from abusing their bodies in the ring.
It has nothing to do with age, just performance. Most didn't think Ric Flair surpassed his peak until the early 90s when he already past 40. On the other hand, Bret Hart had argubly reached his peak at the same age and was never past it up until he retired. It's different for all wrestlers.